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There are varying ways to cost-effectively and clinically-relevantly perform work on wound specimen cultures (1, 2, 3). Most clinical microbiology laboratories will limit full work up of wound specimens to 2 or 3 potential pathogens per culture under normal circumstances. This is not to say that you do not let the physician know about all of the potential pathogens that are grown in the culture, but to fully work up all with identification and antimicrobial susceptibility testing is usually not clinically warranted. Some would stipulate that full work up of all S.aureus, P.aeruginosa & Group A streptococci should always be done regardless of what other organisms are grown in the culture (resident flora or other potential pathogens)(1). Coagulase-negative staphylococci can be important pathogens in surgically collected wound specimens, but rarely are clinically significant in superficially collected specimens unless associated with polymorphonuclear cells on the direct Gram stain, or unless isolated in multiple well-collected specimens (1).
The new 2004 Cumitech 7B published by ASM Press outlines several approaches for the clinically relevant culture work up of lower respiratory specimens (4). Similar approaches can be used for the work up of wound specimens as well (2). One of these approaches, the Q234 System, is outlined below:
The Q234 System:
The Q234 system is "Gram stain directed" which means that more emphasis is placed on organisms seen in the original Gram stain of the specimen. Recall that in order to see 1 organism per oil immersion field on a direct Gram stain, there must be at least 100,000/ml (or g of tissue) of organisms in the specimen (5). Thus, organisms seen in the Gram stain are in higher numbers in the original specimen and are more likely to be associated with infection. You also want to make sure that your specimens are of reasonable quality and show little or no squamous epithelial cell (superficial) contamination.
In the Q234 System protocol if 1 or 2 potential pathogens (PP) are grown in culture, complete identification (ID) and antimicrobial susceptibility testing (AST) is performed. If 4 or more PPs are grown in culture, all isolates are identified based only on spot tests or other rapid testing (morphologically identification [MID]) and no AST is routinely performed. If the culture grows 3 PPs, the work up is based on the correlation of the organisms grown in culture with the organisms seen in the direct Gram stained smear. If 1 or 2 of the 3 PPs grown in culture are seen in the original specimen gram stain, they are worked up with ID/AST. If all 3 of the PPs grown in the culture are observed in the original Gram stain, their work up is limited to MID. Additionally, if the PP(s) are in quantities less than the amounts of resident flora, the PP(s) would have a MID.
This culture work up protocol is Gram stain-directed, limits the identification and susceptibility testing of organisms in mixed cultures, always informs the physician of all potential pathogens grown in the culture, and can be used for standardized work up and reporting of wound culture results (as well as for non-invasive respiratory collections). For this protocol, a statement explaining the limited work up of mixed cultures may be added to culture results: "This is a mixed culture of potential pathogens. Correlation of culture results with the Gram stained direct smear does not help identify any of these isolates as more significant then others. Bacteria may not relate to infection and may represent colonization or contamination." Cultures should be held 7 days to give clinicians an opportunity to discuss additional organism work up if warranted by special circumstances.
NOTE: A modification that would add automatic work up of certain PPs deemed 'always significant' (i.e.; P.aeruginosa, S.aureus, beta-streptococcus, etc.) could be added to the Q234 method.
Q234 SYSTEM:
< 2 PP: ID/AST
3 PP: Look to the Gram stain „³
- ID/AST the 1 or 2 PP you see in the GS
- if all 3 PP are seen in the GS, MID all 3
> 4 PP: MID all PP
* If the PP(s) < in quantity than quantities of normal flora, MID the PP(s).
References:
1. Clinical Microbiology Procedures Handbook, 2nd edition, 2004. Wound and soft tissue cultures. Section 3.13.1; subsection 3.13.1.12
2. Sharp, S. E. 1999. Algorithms for wound specimens. Clin. Microbiol. Newsletter 21(14):118-120.
3. Matkoski, C., S. E. Sharp, and D. L. Kiska. 2006. Evaluation of the Q Score and Q234 Systems for Cost-Effective and Clinically Relevant Interpretation of Wound Cultures. J. Clin. Microbiol. 44:1869-1872.
4. Cumitech 7B. Lower respiratory tract infections. 2004. ASM Press.
5. Clinical Microbiology Procedures Handbook, 2nd edition, 2004. Gram stain. Section 3.2.1; subsections 3.2.1.17 & 3.13.1.15
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